This invention is directed to the field of semiconductor crystal growth, and more particularly to an apparatus for detecting spills of molten material during Czochralski crystal growth processes.
The Czochralski crystal growing technique is commonly used to commercially grow bulk semiconductor single crystals such as silicon crystals. Typically, the semiconductor sub-material used for crystal growth comes in chunks or rocks of varying size up to approximately four inches in diameter. The sub-materials are loaded into a quartz crucible, and the crucible is loaded into the crystal growing apparatus. The semiconductor sub-material is then heated under controlled pressure and temperature, to the point where the sub-material becomes molten, typically above 1425xc2x0 C., and as high at 1450xc2x0 C. The heater used for the process is cylindrical in shape, and surrounds the crucible. Typically, either the heater, the crucible, or both are capable of vertical movement such that heat can be directed at exact locations. Because of this, it is common for the area below the crucible to be many degrees cooler than at the crucible. This temperature variation is further enhanced by an inert vent gas being flushed through the crystal growing apparatus, such that relative cooling of several degrees occurs a very short distance away from the area directly affected by the heater.
During crystal growth, a seed crystal is lowered into the molten sub-material and slowly extracted, thereby generating crystal growth. During extraction, the crucible and the seed crystal are rotated at controlled speeds to promote crystal stability. As molten sub-material is consumed in crystal growth, the crucible is raised to maintain the surface of the molten sub-material in an exact location. The heater is also adjusted to provide heat at the necessary locations.
During the process of melting the sub-material, an operator may monitor the crucible through a viewing port. The view is quite restricted, however, and only allows a view of the top surface of the sub-material, and does not provide a view of the entire crucible. As the sub-material transforms from solid to liquid form however, the operator cannot accurately determine the liquid level of the molten material due to air pockets, floating chunks of sub-material, and the like. As such, there is no accurate way to monitor the volume of liquid contained within the crucible visually until the entire sub-material has become molten and the liquid has reached a state of equilibrium.
In the unlikely event that the crucible has a leak, there currently is no feasible way for the operator to determine that molten sub-material is leaking out of the crucible. A leak of the molten sub-material is a significant safety risk, and must be controlled. Since the growing apparatus and therefore the submaterial is heated above 1425xc2x0 C., if the sub-material were to escape the confines of the crucible it could cause significant damage to the growing apparatus. Further, the apparatus has a water jacket around it for containing the heat within the growing chamber. If the molten sub-material were to breach the water jacket, a steam explosion would occur, causing significant damage to the growing apparatus and surrounding facilities, and very likely would cause significant harm to anyone in close proximity.
Because of the difficulty in monitoring the liquid level in the crucible during the melting process, it is possible to have a large spill of molten material without any indication or warning to the operator. Although a multiplicity of devices exist for detecting a leak in a container such as a hot water heater, these devices are limited in their application and cannot be applied to the detection of melt spills of molten semiconductor at such extreme temperatures.
In U.S. Pat. No. 4,805,662 Moody discloses and apparatus for detecting water leakage underneath a water container such as a hot water heater. This patent describes a leakage collector tray with a peripheral wall that extends upwardly several inches from the base. The collector tray contains a float-operated switch or sensor mounted on the peripheral wall of the leakage collector tray, and the tray is placed underneath the hot water heater. When the water tank develops a leak, water will leak out and collect in the leakage collector tray. Once the water level in the collector tray reaches the float or sensor, an alarm will activate, thereby giving notice of the leak.
Kerbs also discloses an apparatus for detecting water leakage underneath a water container such as a hot water heater in U.S. Pat. No. 5,188,143. This patent describes a flexible elongated sensor strip placed to encircle a ground area beneath the hot water heater. Types of sensors can be varied to require a determined level of water height to activate the alarm.
In U.S. Pat. No. 5,334,973, Furr discloses a three-layered sensor that completely encases a hot water tank, and detects leaks anywhere on the tank. The first layer is nonconductive material that becomes conductive when it absorbs moisture. The second layer is a conductive layer, and the third layer is used as a containment layer that is neither conductive nor absorbent. When a leak occurs, the first layer of the sensor absorbs the moisture and completes an electrical circuit with the second layer, which activates an alarm system.
None of these patents, however, are capable of monitoring molten semiconductor spills due to the extreme environment found within the crystal growing equipment; particularly the extremely high temperatures. Further, each of these leak detection systems are designed to be placed outside of the container, and therefore the liquid must leave the confines of the container to be detected. In the case of crystal growth, these methods could not be utilized due to the safety hazards involved with the elevated temperatures of the molten semiconductor material.
Therefore, a need exists for an apparatus that can detect a melt spill of a molten semiconductor material that can withstand extreme temperatures, be located within the crystal growing apparatus while maintaining cleanliness of the growing environment, and warn an operator of the spill quickly and effectively.
The present invention provides an apparatus for detecting a spill of molten material during the melt preparation and crystal growth steps of semiconductor crystal manufacture. Any spill of molten material is a dangerous situation. Therefore, the embodiments of the present invention relate to detection of a melt spill, regardless of spill size, in a likely area where the spill will be present. Typically, a molten semiconductor material in a growing apparatus is contained within a quartz crucible. The quartz crucible is in turn held by a two-piece graphite susceptor, which in turn sits in a susceptor cup, holding the susceptor pieces together. These parts and the physical area wherein they are housed in the growing apparatus are often called xe2x80x9chotzonexe2x80x9d areas or components. Because of the extreme temperatures involved with the process, the quartz crucible becomes somewhat elastic, and therefore must be held and supported by the graphite susceptor, which can better handle such high temperatures. Since the susceptor, however, is two pieces with a separation running top to bottom, it must be held by the susceptor cup. The susceptor cup has very short side walls, and is primarily used to hold and keep the susceptor together. In this arrangement, if a crucible fails, the leak will run down between the outer wall of the crucible and the inner wall of the susceptor to the bottom of the crucible-susceptor interface. The leaked molten material will then seep through the separation between the two susceptor halves, and flow into the susceptor cup. The susceptor cup is shallow, and is filled with the susceptor itself, therefore not being able to hold or contain material that has leaked out of the crucible. The leaked molten material will then flow out of the susceptor cup and fall to other parts of the crystal growing apparatus, causing damage to everything it touches and potentially causing a very serious safety hazard.
Accordingly, it is among the primary objects of the present invention to provide a novel melt spill detector which is detachably coupled to an audible and/or visual alarm.
Another object of the present invention is to provide a novel melt spill detector which will operate in the high-temperature environment of a crystal growing apparatus without contaminating the crystal growing environment.
Yet another object of the present invention is to provide a novel low-cost melt spill detector that may be placed in any location advantageous to detect the melt spill.
In one embodiment, the sensor or detector includes a layered pad of alternating insulative and conductive material that allows the creation or cessation of an electric signal when the presence of melt spill is detected. The detector may work as a continuous voltage or electrical signal which would be interrupted by the contact or presence of melt spill, or by remaining an open circuit until the conductive melt spill comes in contact with the detector, creating a closed circuit. Upon detection of the melt spill, an audible and/or visual alarm attached to or incorporated into the crystal growing apparatus will warn of the melt spill.
In another embodiment, a thermocouple may be placed in the position where leaked molten material would collect, and detect the increased temperature of the leaked molten material when in contact with the thermocouple. The thermocouple would then trigger an audible and/or visual alarm attached to or incorporated into the crystal growing apparatus, giving warning of the melt spill.
The apparatus according to the embodiments of the present invention can provide immediate detection of the melt spill during the melt down and crystal growing processes. The apparatus is an improved safety feature for the operator and can prevent the costly repair or replacement of a crystal growing apparatus.